There are 15 actinide elements, each with several important isotopes. All actinide isotopes are unstable to radioactive decay involving emission of alpha or beta particles along with gamma rays, as also are all isotopes of the next five atomic numbers below the actinides. Instability and ease of fission of at least some actinide isotopes generally increases with ascending atomic number and spontaneous fission also becomes common in the higher actinides.
All actinide atoms are fissionable, meaning each can be fissioned if its atomic nucleus is struck by a sufficiently energetic neutron. Actinide isotopes can be classified according to whether they are fissile, meaning that they can be fissioned by slow neutrons having room-temperature thermal motion energies of about 0.025 eV. Examples of fissile actinide isotopes include uranium-233, uranium-235, plutonium-239 and plutonium-241, but of these only uranium-235 is found in nature. Only the fissile actinide isotopes can support fission chain reactions, since emitted fission daughter neutrons having enough energy to fission other non-fissile actinides are rare.
Uranium-235 with a 704 million year half-life is the only naturally occurring fissile isotope. Uranium-238, which has a 4.47 billion year half-life is 138 times more abundant and thorium-232, with a 14.05 billion year half-life, is about 500 times more abundant. Both can be fissioned, releasing about 200 MeV of energy per atom. However, they are only fissionable, not fissile. A sustained fission chain reaction is impossible with either of these more plentiful isotopes.
Isotopic enrichment is a difficult industrial process in which a mixture of two or more isotopes of an element is divided into two different mixtures, an “enriched” mixture with an increased concentration of one isotope and a “depleted” mixture with a depressed concentration of the same isotope.
Light water reactors (LWRs), the reactor design currently responsible for producing the majority of the world's nuclear power, rely on the rare uranium-235 isotope as fuel, leaving most of the uranium-238 isotope unused along with the thorium-232 isotope which LWRs entirely ignore. Indeed, the total utilization of mined uranium is only about 1%, with 99% discarded as depleted uranium or as the main component of spent nuclear fuel (SNF). Two alternative physics pathways exist to make use of the two naturally abundant actinide isotopes, uranium-238 and thorium-232, as follows:
Pathway one: Provide a source of sufficiently energetic neutrons to induce fissions without a chain reaction.
Pathway two: Transmute the fissionable isotopes into fissile isotopes, then fission them in a chain reaction.
Initially, there was no known source of fast neutrons with enough generation efficiency to cause net energy release from the first pathway. That changed when the first H-bomb was tested, but for non-explosive applications it remained true that no energy-efficient source of fast neutrons was available.
Isotopes able to be transmuted into fissile isotopes by absorbing a neutron, followed in some cases by beta decay processes, are known as fertile isotopes. All non-fissile actinides are fertile in this sense. Thus, the second pathway for the two fertile and fissionable, but not fissile, natural actinide isotopes is based on the following chained nuclear reaction sequences:

Plutonium-239 and uranium-233 support fission chain reactions as well as natural uranium-235 does.
Every critical nuclear fission reactor incorporating either some uranium-238 or some thorium-232 causes these fissile fuel production reactions to occur. The ratio of the rate of production of new fissile atoms divided by the rate of fissioning fissile atoms is an important reactor parameter termed the Conversion Ratio (CR) if less than unity or the Breeding Ratio (BR) if greater than unity. Typical CR values are 0.6 for LWRs and can exceed 0.9 for a molten salt reactor (MSR) with a graphite moderator. Reaching or exceeding unity would imply converting all fertile atoms into fissile atoms then fissioning them. To exceed unity using uranium-238, it is necessary to use fissile fuel with high plutonium-239 content, minimize neutron captures in structural material, surround the core with an optimized uranium-238 blanket, and frequently recycle the fuel and blanket through a reprocessing center in order to chemically extract bred plutonium from the blanket and insert it into the core while also removing neutron-absorbing fission products.
Experimental Breeder Reactor 1 (EBR-1), the world's first liquid metal cooled fast breeder reactor (LMFBR) began operation in December 1951, producing 200 kW of electricity from its 1.4 MW thermal power. By 1953 it had demonstrated a net breeding gain, thus confirming the conceptual design of a fuel breeder using plutonium fuel with a non-moderating coolant. Much larger LMFBR designs for electricity production are highly constrained but have been built and operated in several countries, all exhibiting BR values slightly exceeding one. In principle such fission breeders could consume most of their actinide feedstock input streams. However, they have not been widely deployed, partly because breeders have higher costs than LWRs both for initial capital outlays and ongoing plutonium fuel recycling, but also due to fears about breeder reactor safety and special breeder concerns about terrorism and weapons proliferation.
Ever since the aforementioned fission breeder design difficulties, costs, and constraints were recognized, there have been efforts to find alternative approaches to harvesting fission energy from the more abundant non-fissile but fissionable actinides. Other than the breeder reactor, the only non-fusion approach ever suggested was Carlo Rubbia's 1995 “Energy Amplifier” which relied on spallation. In nuclear spallation, a beam of very high energy ions emerging from a particle accelerator, typically hydrogen ions with energies between 800 MeV and 7,000 MeV per proton, is focused on a heavy metal target, typically of mercury, lead, or tantalum. Each spallation impact of a very high energy proton on a heavy metal nucleus then sprays out typically 20 to 30 high energy neutrons. However, some protons may fail to cause spallation so the efficiency may not be high. In the Energy Amplifier scheme, high energy spallation neutrons would then cause fissions in thorium-232 or uranium-238 via pathway one, thus releasing even more neutrons which in turn would be absorbed causing pathway two transmutation chains ending in uranium-233 or plutonium-239. A concern about this method is whether the very high energy investment needed per spallation neutron could be offset by the energy content of the fissile fuel produced. Another concern is the high cost and large size of present particle accelerators.
Unlike accelerator driven systems, use of a fusion neutron source may be less of a concern since fusion releases its own nuclear energy. Fusion schemes can be classified according to whether their fusion fuel feeds are deuterium only (DD) or deuterium tritium (DT). If the neutron source is a fusion system using a feedstock of deuterium only, then half of its resulting DD fusion reactions would produce 2.45 MeV neutrons. These do not carry enough energy for pathway one but are adequate for pathway two. If instead a fusion neutron source uses a 50/50 DT feedstock of deuterium and tritium then almost all neutrons produced would be 14.1 MeV neutrons adequate for pathway one. Furthermore, for identical fusion plasma temperature and pressure conditions, the neutron flux will be two orders of magnitude more intense than in the DD fuel case.
While it was recognized in the 1950s that neutron bombardment of fissionable actinide isotopes could greatly expand fissile fuel supplies, there were no controlled fusion neutron sources with adequate energy efficiency. Particle accelerators can easily produce fusion reactions but coulomb scattering is so strong that their typical efficiencies are only about 0.001%. After some initial analyses the subject of hybrid systems was not pursued further. This changed in 1969 when Soviet researchers announced their tokamak device had confined a plasma with temperatures approaching the thermonuclear fusion range. After an international team confirmed the temperatures, other researchers around the world built their own tokamaks and began experiments with fusion-relevant plasmas.
A fusion concept developed in the mid-1970s envisioned a non-Maxwellian ion velocity distribution plasma known as the Two Component Tokamak (TCT). This TCT scheme using neutral beams was the basis for the Tokamak Fusion Test Reactor (TFTR) at the Princeton Plasma Physics Laboratory (PPPL). TFTR performance culminated in a plasma fusion energy gain (Q), i.e., fusion power divided by plasma heating power, of about Q=0.28. Neutral beams were subsequently used at the Joint European Torus (JET) near Oxford in the U.K. to achieve a higher fusion energy gain factor of about Q=0.65. Although these energy gain results are still far too low for pure fusion energy systems, the TCT approach is attractive for Fission Fusion Hybrid (FFH) schemes where the large energy multiplication ratio of the resulting fissions can underwrite the continuous investment of power fed back to a neutral beam plasma heating system. Today it remains the most plausible scheme for producing via fusion the 14.1 MeV neutrons needed for a hybrid nuclear reactor.
A 1974 Lawrence Livermore Laboratory paper reported the results of neutronics simulations of various FFH blanket options using solid materials, including the graded use of various moderators. It predicted an optimized fission blanket power about ten times the DT fusion power with tritium breeding self-sufficiency in the blanket and also a net blanket production of plutonium-239 ranging, for different blanket options, from 2.24 to 4.51 atoms per DT fusion neutron. The report concluded that FFH technology could eliminate the need for isotopic enrichment and could use the then-existing national stockpile of depleted uranium for fuel, thus producing from this source alone a thousand years of electrical power for the US.
This and subsequent published FFH studies envisioned a stand-alone fissile fuel factory which would produce and export plutonium for use in solid fuel rods to be fabricated for and fissioned in other reactors. In proposed FFH systems, fissions only occur in a subcritical fission blanket. In a recent (2009) Gaithersburg, Md. workshop organized by the Department of Energy titled “Research Needs for Fusion-Fission Hybrid Systems,” that limitation was elevated to become a definition: “A fusion-fission hybrid is defined as a subcritical nuclear reactor consisting of a fusion core surrounded by a fission blanket. The fusion core provides an independent source of neutrons, which allows the fission blanket to operate subcritically.”